The phenomenon of superconductivity, that is, zero electrical resistance, possessed by many metals at near absolute zero temperatures, has received steadily increasing attention in recent years due to the development of materials which exhibit this property at sufficiently high temperatures, while carrying relatively high currents in the presence of sufficiently great magnetic fields as to be of commercial utility. Among the more useful of the superconducting materials developed to date is the intermetallic compound Nb.sub.3 Sn. This material has sufficiently good superconductive properties as to render it attractive in the development of useful electrical machinery. However, the manufacture of this intermetallic compound is complicated both by the nature of the compound itself, which is not readily fabricated by simple chemical processes and which is so brittle that the bending of a conductor formed therefrom is substantially precluded, and by the preferred design of conductors using same, which generally have required many individual filaments of the superconductive material to be embedded in a matrix of a non-superconductive material, preferably a metal having high electrical conductivity such as pure Cu.
Recently developed processes for the manufacture of Nb.sub.3 Sn have generally involved the so-called "bronze process", in which rods or wires of Nb are dispersed throughout a matrix consisting of a CuSn bronze. The assembly is worked to a desired final size and heat treated, at which time Nb.sub.3 Sn is formed at the interfaces between the Nb rods and the bronze matrix by diffusion of the Sn from the bronze. See, for example, U.S. Pat. No. 3,918,998. Refinements of the bronze process include providing a quantity of good electrical conductor such as pure Cu in close proximity to the Nb.sub.3 Sn filaments and isolating this pure Cu from diffusion of Sn which would destroy the high electrical conductivity of pure Cu, by interposing a layer of material impermeable to Sn therebetween such as, for example, Ta; see, e.g., U.S. Pat. No. 4,205,199.
A quantity of a good electrical conductor in close proximity to the superconductive material is useful as an alternate current path or shunt in situations where it is likely that some fraction of the superconductive filaments will return to the normally-conducting state, which can happen, for example, in a rapidly-varying magnetic field.
The art, as outlined above, uses the bronze process to achieve multi-filamentary intermetallic superconductors which are stabilized by the provision of a quantity of a good electrical conductor. However, the bronze process is not without its difficulties. Chief among these is the fact that in order to improve the maximum current density carried by the superconductor, it is desirable to increase the amount of superconductive material per unit of cross-sectional area of the whole conductor. To do this it is clear that a sufficiency of tin must be provided, which could be done simply by increasing the percentage of tin in the bronze. Unfortunately, the production of a large number of extremely fine filaments demands a large number of metal-working steps--chiefly drawing--during which the bronze workhardens very quickly, necessitating frequent time-consuming and costly annealing operations. In fact, the practical maximum volume percentage of tin in the bronze which permits working is 15%; and even at this relatively low value, annealing is required roughly every two to six drawing operations, at a rate of 15-20% area reduction per pass.
Thus it can be seen that the bronze process has several drawbacks. The first is the high processing cost due to the frequent annealing required during the reduction (wire drawing). The second is the limitaation on the amount of tin available for the formation of Nb.sub.3 Sn and therefore the limitation on Jc.
The effect of the addition of Al to the Nb core and also to the Cu-Sn bronze matrix, has been reported in the literature, see "Effect of Third Element Additions on the Properties of Bronze-Processed Nb.sub.3 Sn" in Transactions on Magnetics, Vol. MAG-13, No. 1, pp 651-654 (January 1977).
Efforts have also been made to avoid the use of bronze. Chief among these is a method described in U.S. Pat. No. 4,646,428 to William G. Marancik et al, in which a multifilament superconductor is fabricated of the type Nb.sub.3 Sn by carrying out the steps of:
(a) filling the center of one or more copper tubes with Sn and drawing said tubes to form copper-Sn wires;
(b) cabling a plurality of said copper-Sn wires around a core Nb wire thereby forming the basic strand;
(c) bundling a plurality of the cables of step (b) with an eveloping layer of copper which may be in the form of a copper tube, foil or a plurality of finely wound copper wires.
This assembly may be worked up in a number of ways to the final multifilament superconductor, which may involve various cold extrusion and the like. Thus, following step (c) above, the method may be continued by:
(d) drawing the assembly of step (c) to reduce its diameter to a desired size; and
(e) heat treating the product of step (d) to cause Sn to diffuse and form Nb.sub.3 Sn at the surface of the Nb filaments.
In one variation, a plurality of the assemblies formed in step (c) may be inserted in a copper can to form a billet and then the billet drawn to reduce the diameter thereof to a desired size, after which the heat treating diffusion step is carried out.
A diffusion barrier may be used in the first bundling--of step (c)--or in the just mentioned second bundling.
Thus, the centers of one or a plurality of copper tubes are filled with tin to form an initial composite. The percentage of tin in this composite may be freely selected, e.g. in the range of 10-50 weight % but is preferably about 20 weight % tin. Each is suitably sealed at the ends. This is drawn into a wire which is termed Cu-Sn wire.
A plurality of said Cu-Sn wires are cabled around a core Nb wire which may be Nb or predominantly Nb, viz., a NbTi wire containing about 1 weight % Ti.
A plurality of the cables are bundled and inserted in a hollow copper tube, or alternatively wrapped in a copper foil or finely wound with copper wire. The copper tube may be protected by an internal diffusion barrier typically formed of tantalum or of niobium or vanadium.
The resulting filled copper tube is drawn to reduce its diameter in accordance with requirements of the superconducting wire product. Alternatively, a plurality of said filled copper tubes is rebundled and inserted in a hollow copper can--which may also be provided with an internal diffusion barrier--to form a billet, and then the billet, suitably sealed at its ends, is drawn to reduce the diameter thereof to the desired size of the superconducting wire. Either the first bundle or the second bundle or both may be drawn into a hexagonal shape as more fully described in U.S. Pat. No. 4,447,946. The finally drawn wire is heat-treated to cause the Sn to diffuse through said copper wires and form Nb.sub.3 Sn at the surface of the Nb filaments. Typically this involves heating the wire to 550.degree. C. to 750.degree. C., in an inert atmosphere for sufficient time to allow diffusion equilibrium conditions to be established, at which time there is maximum conversion of Nb and Sn to the intermetallic reaction product Nb.sub.3 Sn.
In general, hot extrusion is not used during the process. Extrusion would generally be performed at elevated temperature and pressure resulting in high local temperatures that could cause the tin to melt and form a bronze with the copper, which is unwanted for the reasons above-mentioned. Consequently, the process of said patent is carried out without resort to hot extrusion. However, extrusion at room temperature may be used.
Other variations are also useful. For example, in place of Nb wire, a multifilament Nb in Cu wire may be employed. The initial Cu-Sn composite cabled around the Nb wire can be isolated from other Nb filaments by wrapping this cable with Nb or Ta foil, and this, in turn, can have Cu cabled around it as a stabilizer. A plurality of these assemblies are then bundled and slid into a hollow copper tube as just described. In this way, each Nb filament is very close to a Cu stabilizer thus producing a very stable conductor.
Thus it can be seen that in the initial Cu-Sn composite, the tin is internalized and the process is sometimes referred to as the internal tin process.
An object thereof is to fabricate a Nb wire of very small thickness, for example, about 10 microns in diameter. By using a single Nb wire as a core and cabling around it multiple Cu-Sn wires, a basic strand of small diameter can be produced which, when a plurality of them are provided in a unit area, is adapted to achieve high current density. Tin surrounds each of the core Nb wires and is distributed in an excellent manner to enable it to diffuse to the Nb during the heat treatment step, as well as being supplied in higher amounts, viz., a higher ratio of Sn to Cu, by means of using internalized tin as compared with using bronze. As a consequence of the basic strand concept and the substantial supply of tin, since no area will be completely devoid of Nb.sub.3 Sn-covered Nb filaments, there are more such filaments yielding ultimately a higher current density.
However, one difficulty in practicing the above-described method of U.S. Pat. No. 4,646,428, arises from the mismatch of mechanical properties in the composite material, e.g., the assembly of step (c), which is detrimental to the processing as well as the final product properties. In said method, the Nb, Cu and Sn are co-processed and the softness of the Sn causes problems in the mechanical working and the properties of the final product. This can be seen from the melting points of the several ingredients, as shown below:
______________________________________ Ingredient Melting Point ______________________________________ Sn 231.9.degree. C. (450.degree. F.) Cu 1083.degree. C. Nb 2470.degree. C. ______________________________________